1. Field
This disclosure relates generally to electronic circuits, and more specifically, to circuits for characterizing transistor mismatch.
2. Related Art
MOSFET mismatch is a leading cause of yield loss for analog circuits. Mismatch can vary over time in a manufacturing process, but characterization and on-going monitoring are difficult as they can, if the amount of mismatch is small, require precision and/or time-consuming measurements that are not available or are undesirable in a manufacturing environment.
Direct drain current mismatch measurements, including over bias and geometry, can be used for mismatch characterization via a technique known as backward propagation of variance (BPV). In other approaches, threshold voltage and gain factor mismatch can be used for characterization and/or circuit modeling purposes. Whatever data are used, the test structures for mismatch characterization must be laid out very carefully and the measurements must be done extremely accurately because for precision circuits, mismatches of the order of 0.1% are important (e.g. for a 10 bit data converter, 210=1024 so the least significant bit needs to be accurate to order 0.1%).
Because of the need for highly symmetric device layouts and highly precise measurements, mismatch characterization is difficult and has primarily been done in a lab and not a production test environment. This means that mismatch has often not been tracked over time.